Optical phase-locked loops (OPLLs) are optical devices used in frequency synthesis and in coherent demodulation in optical communication systems for local generation of an optical signal with a frequency and a phase that track those of an optical input signal.
In particular, an OPLL is basically formed by an optical phase detector, an electrical loop filter, and an optical voltage-controlled oscillator (OVCO) comprising a non-modulated optical source, an electrical voltage-controlled oscillator, and an optical amplitude modulator, which is designed to modulate the optical carrier supplied by the non-modulated optical source with the electrical modulating signal supplied by the electrical voltage-controlled oscillator.
In particular, the phase detector receives an optical signal to be locked and a locked optical signal—i.e., one having a frequency and a phase “locked” to those of the optical input signal—, which is supplied by the OVCO, and supplies an electrical error signal indicating the difference of phase existing between the optical input signals.
The electrical error signal generated by the phase detector is supplied to the loop filter, which has a transfer function of a low-pass type and supplies a filtered electrical error signal that is then supplied to the OVCO, which supplies the aforementioned locked optical signal, the instantaneous frequency of which varies proportionally with the amplitude of the filtered electrical signal.
OVCOs are generally obtained with solid-state laser or tunable-semiconductor laser that can be modulated directly, which, although used in the past, present, however, certain drawbacks that markedly condition the use of the OPLLs in which they are inserted.
In particular, although presenting undoubted qualities in terms of spectral efficiency and functionality (insensitivity to non-linear effects) deriving from the reduced linewidth of the solid-state lasers, OPLLs that use OVCOs based upon solid-state lasers are, however, difficult to apply to optical communications systems, in so far as it is somewhat difficult to find solid-state lasers functioning in the frequency grid set down by ITU (International Telecommunication Union). In addition, solid-state lasers are very voluminous and cumbersome, require a lot of power for their operation and are more costly than OPLLs that use OVCOs based upon semiconductor lasers.
OPLLs that use OVCOs based upon semiconductor lasers, although considerably less costly than OPLLs that use OVCOs based upon solid-state lasers, require, however, the use of a distributed-feedback (DFB) technology, which requires the use of wide-band electronic feedback circuits on account of the considerable linewidth of directly controlled semiconductor lasers, and an injection current that is extremely high on account of the non-ideal operation of said devices.
The constant market request for increasingly high data-transmission rates means that the high spectral efficiency and the insensitivity to the non-linear effects of OPLLs will constitute a fundamental factor in next-generation optical communication systems. In fact, from an observation of the evolution of current transmission systems, it may immediately be noted that the performance of standard intensity-modulation direct-detection (IM-DD) transmission systems based upon the “no return to zero” (NRZ) format or the “return to zero” (RZ) format are increasingly approaching the theoretical limits in terms of spectral efficiency and insensitivity to non-linear effects. For these reasons, in order to improve the performance of optical communications systems, the only solution that can currently be pursued would appear to be that of a considerable modification of the structure of the transmission system, for example using, in transmission, phase, frequency, and amplitude modulations and possible combinations thereof, such as, for example, phase shift keying (PSK), frequency shift keying (FSK), quadrature amplitude modulation (QAM), and, in reception, coherent homodyne detection.
By way of example, a PSK binary transmission system with coherent homodyne detection has a sensitivity that is better by 3.5 dB than a standard IM-DD transmission system with NRZ format. Said advantage may be used to reduce by approximately 3.5 dB the mean optical power required for each transmission channel. In terms of peak power there is hence obtained a reduction of approximately 6.5 dB, with consequent drastic reduction in the non-linear effects in the fibre, which are a source of degradation of the performance.
As further example, a 4-PSK transmission system has a spectral occupation that is half that of a standard binary transmission system with NRZ format.
In the literature there exist OPLLs that endeavour to overcome the above drawbacks and are based upon subcarrier modulation, such as, for example, the oscillator optical described in the patent application No. EP 1673883.